The world's population is projected to rise from the current 7 billion to over 9 billion in the next 40 years, and a parallel increase in global food stocks has become a major challenge in the near future. Rice is a major staple crop feeding more human population than any other crops, and its yield must be increased by at least 40% in order to meet the world's demand for food production. However, the rice yield is close to its upper limit in major rice production countries (IRRI, 2010). Additionally, global climate changes, such as rising temperature and water scarcity, further aggravate the stability of rice production.
The grain yield potential in rice is determined by both genetic and environmental factors (Curtis et al., 2005; Wang and Li, 2005, 2006, 2008; Jeon et al., 2011; Yadav et al., 2011). Examples of regulatory genes identified in rice include LAX (a basic helix-loop-helix transcription factor) which controls shoot and panicle branching (Komatsu et al., 2003), Gn1a (a cytokinin oxidase/dehydrogenase, OsCKX2) which degrades cytokinin in inflorescence meristems and increases grain number (Ashikari et al., 2005), FZP (an ethylene responsive element-binding factor) controls the transformation of floral meristems to inflorescent shoots (Yi et al., 2005), DEP2 (dense and erect panicle 2) (Li et al., 2010) and DEP3 (a patatin-like phospholipase A2 superfamily domain-containing protein) (Qiao et al., 2011) controls panicle morphology, SPL14 (the SQUAMOSA promoter-binding-like protein) controls tiller and panicle developments (Jiao et al., 2010; Miura et al., 2010), and GS5 (a putative serine carboxypeptidase) which controls grain size (Li et al., 2011).
Plant hormones play crucial roles in the regulation of plant architecture and grain yield, and gibberellins (GAs) are a class of essential hormones that control seed germination, plant height, root growth, flowering and seed production (Carrera et al., 2000; Lo et al., 2008; Dayan et al., 2010; Jia et al., 2011). Production and maintenance of optimal levels of bioactive GAs are important for plant normal growth and development. Slight reduction in GA levels results in semi-dwarfism of plant but that are more lodging-resistant and improve the harvest index (the ratio of grain weight to total weight of grains plus straws) (Khush, 1999). Breeding of semi-dwarf wheat and rice cultivars by incorporation of two genes, the Reduced height 1 (Rht1) and semi-dwarf (sd1) that are involved in GA signaling and biosynthesis in wheat and rice, respectively, and with the combination of N-fertilizer application, led to quantum leap of yield increase in the two cereal crops, and that is the basis behind the so called “Green revolution” (Khush, 1999; Peng et al., 1999; Sasaki et al., 2002; Spielmeyer et al., 2002b, a; Botwright et al., 2005b, a).
Recent genetic, biochemical, and structural studies have significantly enhanced our knowledge on biochemical pathways of GA biosynthesis and catabolism, genes and enzymes involved in these pathways and the molecular mechanism of GA signaling in plants (Hartweck, 2008; Sun, 2008; Yamaguchi, 2008; Hedden and Thomas, 2012). GA 3-oxidase (GA3ox) and GA 20-oxidase (GA20ox) are essential enzymes in biosynthesis and GA 2-oxidase (GA2ox) in inactivation of GA metabolites that determines final concentrations of bioactive GA (GA1, GA3, GA4, and GA7) (Hedden and Phillips, 2000).
A major catabolic pathway for GAs is initiated by a 2β-hydroxylation reaction catalyzed by GA2ox. The class C19 GA2oxs more commonly found in various plant species hydroxylate the C-2 of active C19-GAs (GA1 and GA4) or C19-GA precursors (GA20 and GA9) to produce biologically inactive GAs (Sakamoto et al., 2004). A class of C20 GA2oxs, including Arabidopsis GA2ox7 and GA2ox8, spinach GA2ox3 and rice GA2ox5, GA2ox6, and GA2ox9 that specifically hydroxylate C20-GA precursors, are relatively rare and less studied compared with C19 GA2oxs (Schomburg et al., 2003; Lee and Zeevaart, 2005; Lo et al., 2008). Class C20 GA2oxs contain three unique and conserved amino acid motifs that are absent in class C19 GA2oxs (Lee and Zeevaart, 2005; Lo et al., 2008).
Plant architecture, such as plant height, tiller number, and root system, has been important agronomic traits for breeding. Manipulation of GA levels offers an opportunity for further improvement of plant architecture for optimal grain yield potential. GA biosynthesis and catabolism enzymes have been used to control the endogenous level of bioactive GA in transgenic plants. For examples, overexpression of a GA catabolic enzyme, GA2ox1, controlled by a under the control of a GA biosynthesis gene (GA3ox2) promoter results in semi-dwarf transgenic rice with normal in flowering and grain development (Mohanty et al., 2002). Over-expression of a mutated class C19 GA2ox6 under the control of ubiquitin promoter produces semi-dwarf transgenic rice with increased tiller number and root system (Abiko et al., 2008). However, constitutive overexpression of GA2oxs normally leads to severe dwarf sterile in transgenic plants (Sakai et al., 2003; Sakamoto et al., 2004). Consequently, different strategies were used to overexpress some of these enzymes. Overexpression of C20 GA2oxs seem to offer more beneficial effects on plant growth and architecture, such as bearing more seeds and producing earlier and more tillers and stronger roots and stems as compared with C19 GA2oxs (Lo et al., 2008). Deletion of the conserved motif III in C20 GA2oxs further improves plant architecture and seed production as compared with the wild type C20 GA2oxs in transgenic rice (Lo et al., 2008).
There is a need to fine tune the GA level in plants to retain desired GA deficient advantages, with reduced or no unfavorable defects.